The present invention relates generally to a deflecting yoke suited for use in a picture tube of a projection color television receiver set. In more particular, the invention concerns the deflecting yokes used in picture tubes adapted for projecting light images to a screen in the directions inclined relative to the screen in a projection color television receiver set which comprises three picture tubes, i.e. a red light emitting picture tube, a green light emitting picture tube and a blue light emitting picture tube, wherein light images in respective colors produced by these three picture tubes are projected to a single screen to thereby reproduce a synthesized color picture on the screen.
There has been known a projection color television receiver set which comprises a first picture tube provided with a red light emitting phosphor (hereinafter referred to also as the red picture tube), a second picture tube provided with a green light emitting phosphor (hereinafter referred to also as the green picture tube) and a third picture tube provided with a blue light emitting phosphor (hereinafter referred to also as the blue picture tube), wherein light images or pictures reproduced on face plates of these picture tubes, respectively, are projected onto a single screen through projection lenses disposed, respectively, in front of the associated face plates, to thereby reproduce or reconstruct a color picture on the screen. In the projection color television receiver set of this type, the three picture tubes are disposed in juxtaposition to one another. For example, the green picture tube for producing the green light image is disposed at the center of the juxtaposed array, while the red picture tube for producing the red light image is located on one side of the green picture tube with the blue picture tube being positioned on the other side. In the projection color television receiver set of such arrangement, the light images produced by the red picture tube and the green picture tube which are latterally disposed are projected onto the screen in the directions inclined relative to the plane of the screen, because the face plates of these picture tubes are positioned at an angle to the screen. For this reason, the images or pictures produced by the blue picture tube and the red picture tube are difficult to be converged and reproduced on the screen in the correct patterns or forms. On the other hand, the image produced by the green picture tube disposed at the center of the juxtaposed tube array can be reproduced on the screen in a relatively correct form without any appreciable deformation. More particularly, reference is to be made to FIG. 1 of the accompanying drawings. An electron beam emitted by an electron gun 1c is deflected by a deflecting yoke 1b, whereby a raster of a rectangular form is produced on a faceplace 1a of a picture tube 1 by the deflected electron beam. When the faceplate 1a of the picture tube 1 is disposed in parallel with the screen 4, the rectangular raster 5 produced on the faceplate 1a is projected onto a screen 4 through a lens system 2 in a raster 6 of a corresponding rectangular form with a given magnification, as is illustrated in FIG. 3. On the other hand, when the faceplate 1a of the picture tube 1 is disposed with an inclination to the screen 4 at an angle .theta. as shown in FIG. 2, the raster 5 produced on the faceplate 1a of the picture tube 1 is deformed to a raster 7 of a trapezoidal form when projected onto the screen 4, as can be seen in FIG. 3. In this connection, it will be noted that when the picture tube 1 is disposed on the righthand side relative to the center 0 of the screen 1 as viewed in FIG. 2, the magnified raster 7 projected on the screen 4 is so deformed as to have a long left side and a short right side, as is illustrated in FIG. 3. On the other hand, when the picture tube 1 is disposed on the left side relative to the center 0 of the screen 4, the raster 7 on the screen 4 has of course a short left side and a long right side. Such difference in the form of the rasters projected on the screen 4 will naturally give rise to the problem of misconvergence in the projection color television receiver set.
Further, when the surface of the screen 4 facing toward the projector 1 is curved spherically as indicated by a broken line 4a in FIG. 2, vertical lines 7a of the raster 7 on the screen 4 become arcuate, as is illustrated in FIG. 3. Such arcuate vertical lines 7a also bring about the problem of misconvergence.
In order that the raster making appearance on the screen be of the correct rectangular form in the projection system of the above mentioned arrangement, the raster on the face plate 1a of the picture 1 will have to be reproduced, for example, with a deformation reverse or complementary to that of the raster 7 produced on the screen 4 with the vertical raster lines produced on the faceplate 1a being also curved in the direction opposite to that of the raster lines 7a, so that deformation or distortion of the raster produced on the faceplate 1a of the picture tube 1 is cancelled out upon being projected onto the screen 4. In the hitherto known projection color television receiver set, generation of the raster of the complemental trapezoidal form having complementarily arcuated vertical lines on the faceplate of the picture tube 1 is accomplished, for example, by resorting to the use of an auxiliary deflecting yoke and an associated electrical circuit for supplying a current to the auxiliary deflecting yoke. However, such electrical circuit can usually be realized only in an extremely complicated configuration and nevertheless suffers from a serious drawback that the performance of the circuit is very susceptible to variations under influence of variations in the source voltage, for example, as the result of which the form of the raster reproduced on the faceplate 1a of the picture tube 1 and hence the form of the raster 7 projected on the screen 4 undergoes corresponding variations or distortions, involving again the problem of misconvergence. Further, linearity of the reproduced picture is adversely influenced. For these reasons, it has been difficult to produce the picture of a satisfactory quality on the screen 4.